Known liquid crystal devices comprise two glass plate electrodes sealed together with a glass frit and a layer of liquid crystal material between them. The spacing between the plates is generally on the order of about 0.5 to 1 mil (13 to 25 microns). In order to provide devices having long life, the seal must be completely hermetic to prevent any impurities, particularly water or oxygen, from entering the cell and degrading the liquid crystal material, and to prevent leakage of the liquid crystal material, causing air bubbles which would detract from the appearance of the device.
The problem of obtaining a good seal is not simple any many prior art workers have devoted themselves to obtaining liquid crystal devices with hermetic seals by a reproducible, low cost process.
After the glass plates have been assembled, an opening or port must be available to fill the enclosure with the liquid crystal composition. Formerly, one or two holes were drilled in one of the plates leading to the interior spacing to provide such ports. After the cell was filled with the liquid crystal material, the ports were plugged with metal or epoxy. This method is unsatisfactory because it results in high glass breakage and the plug tends to be pushed out by any pressure inside the cell, such as that created by exposure to elevated temperatures. Further, it is extremely difficult to get a completely hermetic seal due to differences in expansion coefficients and the like between the glass and the metal or the plastic plug used.
In more recent processes, a gap spacing in the glass frit used to cement the plates together has been used to form the fill port for the liquid crystal material. This gap must then be sealed after the liquid crystal is introduced into the cell. Such a process is described in U.S. Pat. No. 3,751,137, which teaches that a conductive epoxy may be used to seal the cell; or an indium solder may be applied directly; or a metal film can be applied to the edge of the opening and covered by a solder. However, none of these sealing techniques has proven to be satisfactory because they do not completely hermetically seal the cells.
British Patent No. 1,381,428 describes closing a gap in a frit seal by providing a metal coating around the gap and soldering over the metal.
British Patent No. 1,381,077 describes another method for closing a gap in the frit seal whereby a vapor deposit of three metals, chromium, copper and gold, is sequentially applied to the opening which is then covered by solder. This method also has proven unsatisfactory because of strength and thermal expansion differences with the glass. Further, it is difficult to prevent the metal particles being vapor deposited from entering the cell, wherein they can aggregate causing shorts between the electrodes or unsightly metal deposits.